1. Field of the Invention
The present invention concerns a method for free spraying of the nozzles of an inkjet print head (spraying the nozzles of an inkjet print head to clear those nozzles of clogging), in particular in a franking and/or addressing machine.
2. Description of the Prior Art
It is worthwhile to use the advantages of ink printing in the field of machine franking and/or addressing Printing in such devices ensues without contact by means of an inkjet print head (see, for example, DE 44 24 771C1 and EP 0 696 509 B1). Ink printing technology has the disadvantage, however, that often more ink is consumed for the cleaning of the inkjet print head than for the printing process. That is particularly serious in the case of individualized printing (as opposed to continuous printing.
A franking machine with an inkjet print head is known (see EP 0 696 509 B1 and U.S. Pat. No. 5,806,994) in which the letters are transported lying horizontally and the nozzle surface is arranged parallel to the moving letters. In this machine the nozzles that are used less frequently or not at all are freely sprayed upon printing, as long as no letter is present in front of the print head. For this purpose the letter transport device is provided with corresponding recesses, and a capture reservoir for the freely sprayed ink is arranged below these recesses. The ink consumption is reduced in this manner relative to the priming, but the horizontal letter transport and corresponding design measures are a requirement for achieving this result.
As used herein, “free spraying” means a one-time or repeated activation of one or more nozzles. In contrast to this, “priming” means a multiple, successive free spraying of all nozzles Free spraying is consequently the ink-saving form of the cleaning of the nozzles of an inkjet print head.
An ink printer with a variable cleaning algorithm is known (see EP 0 934 828 A2) in which past franking imprints are recorded and evaluated (historical log) and from log this cleaning regime (schedules) is derived. Information about maximum downtime, weekly letter arrival as well as number and type of the processing workflows per week are recorded in the historical log. The type of the imprints and thus the actual activation of the individual nozzles are not taken into account.
A device for cleaning an inkjet print head in a franking and/or addressing machine is known (see DE 10 2005 052 150.9-27) in which the inkjet print head is stationary, but can be pivoted behind a guide plate in a print window. During a cleaning procedure, the device seals the printhead orifices from the environment. By means of a transport device the print medium is caused to rest against the guide plate (tilted beyond the vertical) and is transported standing on an edge Like the inkjet print head behind the guide plate, the cleaning and sealing device is arranged such that it can be displaced onto and away from the same guide plate. By means of associated displacement mechanism, the inkjet print head is alternatively pivotable into a printing position or into various cleaning regions as well as into a sealing position.
In the printing position the nozzle surface of the inkjet print head is arranged parallel to the guide plate and thus also parallel to the print medium.
In a first cleaning region, the inkjet print head is pivoted out from the printing position to an extent so that at a separation exists that is at least double the normal separation from the print medium, but all ink jets still reach the print medium. Use is made of the fact that, at the provided double separation, the inkjet printing drops disintegrate into smaller satellite drops, whose scatter region is so large that a recognizable print pattern is no longer present.
Given higher transport speeds for the print medium, this method can be applied only in a limited manner due to the mass inertia of the inkjet print head that must be pivoted. The speed of the rotation movement is also limited in order to avoid an unwanted flinging of ink due to the pivoting.